Wheelie bars are often used in race vehicles, which experience rapid acceleration, to prevent the front end of the race vehicle from lifting excessively. These devices usually include two wheelie wheels, located behind the rear wheel(s), which are each connected to an axle bracket near the rear wheel(s) of the vehicle. The wheelie wheels are connected together by a common wheelie wheel axle. The axle bracket is usually connected by two connecting bars to the portion of the chassis in front of the rear wheel. One of these connecting bars is usually connected to the axle bracket above the center of the rear wheel, while the other is connected at a point below the center of the rear wheel. This type of connection is termed a 4-link or 4-bar suspension. When the front end of the race vehicle accelerates and the front wheels are lifted from the ground, the axle bracket rotates slightly, causing the lower bar to be in compression, and the upper bar to be in tension. When the front wheels return to the ground, because one bar is in tension and the other bar is in compression, the chassis violently reacts to the sudden reduction in load, making it difficult to control the vehicle. This reaction is termed "chassis flex."
Another shortcoming of the 4-link connection system is that it concentrates the load associated with lifting of the front wheels on one portion of the chassis: that part directly in front of the rear wheel. Because the load is applied at one point, this part of the chassis is prone to losing its structural integrity. Therefore, the structural members adjacent to the connection point must be strengthened in order to disperse the load throughout the frame structure.
Finally, with conventional 4-link connection systems, the wheelie wheels bear a significant portion of the load of the vehicle, which reduces the traction of the rear wheel, thus reducing the energy that may be transferred to the pavement and reducing the acceleration of the vehicle.